Elevator cars require suspension systems to position the car laterally in the hoistway and to cushion disturbances to the car due to load imbalance and passenger motion. The present state of the art employs a T-shaped steel guide rail 10 fitted by a bracket 12 to the side of the hoistway, as shown in FIGS. 1 and 2, and rolling wheel guides 14 as shown in FIG. 3, fixed to a car on compliant mountings, as shown in FIG. 4. The wheels roll on the guide rails to maintain the lateral position of the car and to cushion disturbance forces imposed on the car.
Forces imposed on the car are resolved into two components, named front-to-back, and side-to-side. Separate wheels are used to act in each of these directions. Side-to-side force is developed by a wheel 16 pushing against the narrow inner surface at a distal end of the rail, and front-to-back force is developed by a separate wheel 18 pushing against the broad side surface of the rail.
A limitation of the existing passive roller guide suspension systems is that irregularities in the rails are transmitted to the car frame, resulting in unwanted noise and vibration for the passengers.
An alternative approach to lateral suspension of the car is to employ actively controlled electromagnets fixed to the car to develop a force of attraction between the actuator and the guide rail. Active control of the magnets is used to offset the static forces due to load imbalance, and to provide dynamic forces to cushion disturbances. See, for example, published European Patent Application 0 467 673 A2. Active magnetic suspension is a non-contacting lateral suspension scheme that is smoother and quieter than the existing roller guide technology, and offers improved elevator ride quality. Although previous disclosures have shown electromagnet actuators, the geometry employed has not been optimized, particularly for the side-to-side actuation.
The prior art in magnetic guidance of elevator cages guided by T-shaped rails employs separate actuators for front-to-back and for side-to-side motion, at the bottom two corners or even at each of the four top and bottom corners of the elevator cage. For example, in U.S. Pat. No. 4,754,849, this will involve three separate cores and coils at each of the four corners of the elevator cage.